In scattered red light measurements, an assembly of suspended particles, e.g. in a cuvette, is illuminated by coherent light. The resulting pattern of scattered light is focused on a light sensitive multi-element detector, which transforms light into electrical signals, From these digitized signals the particle size distribution is calculated by a computer via the solution of a Fredholm integral equation.
Devices for the determination of particle size distributions by mathematical analysis of measured scattered light intensities utilize, among others, laser diffraction instruments or Fraunhofer diffraction instruments. There, an assembly of fine particles is illuminated by monochromatic light of a laser or a laser diode. Each individual illuminated particle of the size x produces a pattern M(x,.crclbar.) of scattered light, where .crclbar. is the scattering angle. M(x,.crclbar.) is either known from theory (Fraunhofer approximation or Mie theory) or can be determined experimentally. The pattern of scattered light being produced by an assembly of particles is the superposition of the patterns of all individual particles. That pattern, depending on the scattering angle .crclbar., can be registered as light intensity I(.crclbar.) by a device which is described below.
The computation of the to be measured particle size distribution Q(x) of the assembly of particles is carried out by the solution of a Fredholm integral equation by a computer, which is connected to the registration device. Another known mathematical method is the regularisation according to Phillips-Twomey.
The essistential part of laser diffraction instruments is the device for the registration of the scattered light pattern. Two different methods are being realized. In one setup, the scattered light pattern is focused by an objective on a multi-element detector and is registered there. This multi-element detector is e.g. a ring detector with concentric circular light sensitive segments. This detector converts the light, which is scattered to different angles, into electrical signals, which are associated to the individual angular zones of the scattered light, e.g. in a known design with a 32-element-detector. In another known setup, the scattered light pattern is focused on a plane where a rotating mask with several apertures is situated, which transmits light from different angular sections at different times. These apertures, e.g. rectangular apertures, are positioned at different radial distances from the axis. They are staggered in circumferential direction. The light transmitted through an aperture and through an additional stationary slit is focused on a one-element detector. Consequently, this detector registers at different times the light scattered into different angular sections.
Also other devices are known, where the objective is not arranged behind the cuvette or the assembly of particles, respectively, in front of the multi-element detector or the rotating mask, but between the light source and the cuvette or the assembly of particles, respectively. With this variant it is possible with simple means to detect the scattered light also at large angles.
The known devices have mainly three disadvantages.
They are complicated. If a multi-element detector is used, then each element of the detector is connected with electrical amplifier for the signal. If a rotating mask is used, then its actual angular position has to be monitored by additional electrical signals.
The devices known to date require a precise mechanical adjustment of the optical components. The optical axis of the arrangement (illumination-, focusing- and detection device) has to agree with high precision with the center of the multi-element detector or the rotating mask, respectively. In one known device this problem is solved by a motor-controlled adjustment.
The use of an objective requires that all rays which are scattered from the particles into one direction are focused to one single point in the focal plane. A rotationally symmetric pattern of scattered light is produced; from its angular scattered light distribution the particle size distribution can be computed, which produced this pattern. This requires, that the rotationally symmetric scattered light pattern must be concentric relative to the optical axis.
The devices known up to this time detect the scattered light intensity only at a restricted number of angles, e.g. 32 in the device with a 32-element detector. In other devices the number of angles is even less. Consequently, the available information for the computation of the particle size distribution is restricted, which can have negative effects on the accuracy of the computed particle size distribution from the scattered light pattern.
It is the object of the invention to provide a device of the above-mentioned kind which can be easily assembled and to provide a method for the determination of particle size distributions with higher accuracy, even if the distribution is broad or extended to the submicron range.